1. Field of the Invention
The present invention relates to a projector and a method of projecting an image from the projector, and more particularly, to a projector which corrects an image projected obliquely upward toward a vertical screen for a resulting keystone distortion, and a method of projecting an image from the projector.
2. Description of the Related Art
A projector projects an image displayed on a liquid crystal panel or the like onto a screen through a projection lens. When the optical axis of the projection lens is perpendicular to the screen, the image on the liquid crystal panel is enlarged and projected onto the screen without distortion. However, when the optical axis of the projection lens inclines with respect to the screen, a projected image will suffer from a keystone distortion. Therefore, a keystone distortion correction is required for correcting the image on the liquid crystal panel before the image is displayed. Recent projectors generally have such a keystone distortion correcting function.
A conventional keystone distortion correcting method is disclosed, for example, in Japanese Patent Laid-open Publication No. 249401/2001.
When a rectangular image on a liquid crystal panel is projected onto an upright screen obliquely upward from a projector, i.e., with the optical axis of a projection lens oriented obliquely upward, image 11 of an upward diverging trapezoid is projected onto screen 10, as illustrated in FIG. 1.
Assume that longer edge 13, which is the upper edge of image 11, is longer than shorter edge 12, which is the lower edge of image 11, by a length of K on both sides (the longer edge and shorter edge refer to two parallel longer and shorter edges of a trapezoid except for oblique edges). In this scenario, image 11 is corrected for the keystone distortion by correcting the image displayed on the liquid crystal panel. As a result, an image of a rectangle is displayed on screen 10, as illustrated in FIG. 2, with upper edge 13 being cut away by K on both sides thereof.
The keystone distortion correction disclosed in Japanese Patent Laid-open Publication No. 249401/2001 fixes the lower edge of an image for the correction, whereas another keystone distortion correction is made with the upper edge of an image being fixed. The keystone distortion correction which involves fixing the lower edge facilitates adjustments. On the other hand, the keystone distortion correction which involves fixing the upper edge can take a larger projection angle (an angle of a line which connects the intersection of diagonals of an image on the screen to a projector with respect to a horizontal plane after adjusted), and can readily display an image at a higher position on the screen. Determination on either of the keystone distortion corrections to employ depends on manufacturers and types of projectors.
FIGS. 3A to 3C show changes in an image on a liquid crystal panel resulting from the keystone distortion correction which is made with the lower edge being fixed, when the image is projected onto an upright screen from a projector which is installed obliquely upward to the screen.
Image 15 on the liquid crystal panel before the keystone distortion correction appears as a rectangle, as illustrated in FIG. 3A. FIG. 3B illustrates image 16 on the liquid crystal panel when image 15 undergoes the keystone distortion correction (image 15 is superimposed on image 16. This applies as well to FIGS. 3C and 5A to 5C), and FIG. 3C illustrates image 17 on the liquid crystal panel when a larger amount of correction is applied. Since a projected image which is distorted into a trapezoid is made wider in the upper edge and longer in the vertical direction, the corrected image has the upper edge reduced in width and the upper edge brought downward, while the lower edge remains at a fixed position.
FIG. 4 shows changes in an image on a screen resulting from a lower edge basis keystone distortion correction, which is made with the lower edge being fixed, when the image is projected onto such an upright screen from a projector which is installed obliquely upward to the screen. Image 13 which suffers from a keystone distortion before the correction is corrected into image 20 by a certain degree of the keystone distortion correction, and rectangular image 21 is reached by adjusting image 13 with a larger amount of correction. The keystone distortion correction causes the image to shift downward, so that when image 13 is first expanded over the entire displayable area of the screen, image 21 corrected for the keystone distortion appears in a lower portion of the displayable area.
With the lower edge basis keystone distortion correction, an adjustment does not cause any change in the position and width of the lower edge of an image. Therefore, an user may first bring the lower edge of an image to a target position and set the width of the image to a target width before the keystone distortion correction is made, to readily produce a normal rectangle with the lower edge located at the target position and having the target width.
However, although a projector is inherently inclined upward for projection in order to display an image at a higher position, the lower edge basis keystone distortion correction results in an image which is displayed in a lower portion of a displayable area on the screen. Thus, the lower edge basis keystone distortion correction is disadvantageous over an upper edge basis keystone distortion correction, described below, in that images are displayed only at low positions.
FIGS. 5A to 5C show changes in an image on a liquid crystal panel, resulting from the keystone distortion correction which is made with the upper edge being fixed, when the image is projected onto an upright screen from a projector which is installed obliquely upward to the screen.
Image 15 on the liquid crystal panel before the keystone distortion correction appears as a rectangle, as illustrated in FIG. 5A. FIG. 5B illustrates image 18 on the liquid crystal panel when image 15 undergoes the keystone distortion correction, and FIG. 5C illustrates image 19 on the liquid crystal panel when a larger amount of correction is applied. While the width of the upper edge is narrowed and the lower edge is brought upward, the upper edge remains at a fixed position.
FIG. 6 shows changes in the image on the screen resulting from the upper edge basis keystone distortion correction, which is made with the upper edge being fixed, when the image is projected onto such an upright screen from a projector which is installed obliquely upward to the screen. Image 13 which suffers from a keystone distortion before the correction is corrected into image 22 by a certain degree of the keystone distortion correction, and rectangular image 23 is reached by adjusting image 13 with a larger amount of correction. The keystone distortion correction causes the image to shift upward, so that when image 13 is first expanded over the entire displayable area of the screen, image 23 corrected for the keystone distortion appears in an upper portion of the displayable area. In other words, a larger projection angle is ensured.
The upper edge basis keystone distortion correction causes a change in the position of the lower edge of an image and in the width of the upper edge of the image depending on the amount of adjustment. Therefore, when an user sets the lower edge of the image at a target position, the user must again set the position of the lower edge each time the keystone distortion correction is made. On the other hand, when the user sets the upper edge of the image at a target position and width, the user must alternately set the width of upper edge of the image by adjusting a zoom lens and make the keystone distortion correction, because each keystone distortion correction causes a change in the width of the upper edge, thus burdening the user with complicated operations. However, since the image appears in an upper portion of the displayable area, the image can be displayed at a high position.
Therefore, in regard to the manual adjustment described above, the keystone distortion correction which involves fixing the lower edge is more user-friendly if the operability is given a higher priority. On the other hand, as shown in a projector apparatus disclosed in Japanese Patent Laid-open Publication No. 339671/2001, an automatic angle adjustment has been more generally employed in recent years. The automatic angle adjustment involves detecting a tilt angle of a projector using an acceleration sensor or the like, and automatically making an adjustment based on a keystone distortion correction in accordance with the tilt angle of the projector. The automatic angle adjustment even eliminates a user interface itself.
While there has been developed a projector which has an automatic angle adjusting function and permits the user to select the automatic angle adjustment or manual adjustment, the conventional projector makes only one of the keystone distortion correction which involves fixing the lower edge of an image and the keystone distortion correction which involves fixing the upper edge of the image.
The keystone distortion correction which involves fixing the lower edge of an image cannot take a large projection angle and therefore cannot display the image at a high position on a screen or the like. On the other hand, the manually adjusted keystone distortion correction which involves fixing the upper edge of an image disadvantageously burdens the user with complicated operations. Even a projector which has an automatic angle adjusting function will suffer from a similar disadvantage when the manual adjustment is selected.
Therefore, if the manual adjustment for the keystone distortion correction is selected, the lower edge basis keystone distortion correction should be employed considering the high operability. However, If the automatic angle adjustment is selected, the upper edge basis keystone distortion correction should be employed because of its aptitude for ensuring a large projection angle and the fact that the user interface itself is made unnecessary by the automatic angle adjustment which detects a tilt angle of a projector by an acceleration sensor or the like to automatically make a keystone distortion correction.